1. Field of the Invention
The invention is generally related to methods for manufacturing multilayer ceramic substrates and, more particularly, to a method for forming multilayer ceramic substrates with cavities extending through the substrate from the top to bottom where the internal dimensions of the cavity are precisely controlled.
2. Description of the Prior Art
Multilayer ceramic structures are often formed by a "sub-lamination" process wherein the first two ceramic sheets are stacked on top of each other and laminated at low temperature and pressure, and then the next two ceramic sheets are stacked on the top and bottom of the first two ceramic sheets with the four sheets then being laminated together at low temperature and pressure, and so on, until the total number of layers in the multilayer ceramic structure is reached. One problem with the "sub-lamination" process is that it requires multiple lamination steps, which slow product throughput. Another problem with the "sub-lamination" process is that it does not provide uniform lamination from the first layer to the last. This is primarily due to the first set of layers being exposed to pressure and temperature more than the last set. This in turn causes non-uniform shrinkage of the ceramic layers during the sintering cycle, and results in excessive camber of the part, and may also result in cracking of the ceramic material.
The "sublamination" process is not ideal for forming open-centered multilayer ceramic substrates wherein a chip/heat sink combination will be connected to the substrate in a cavity formed through the substrate. The process results in non-uniform shrinkage of the layers, and this causes excessive camber at each of the individual layers as well as adversely effects the overall flatness of the part. In addition, when the layers are warped, it is difficult to make electrical connections to the wire bond pad areas. When the part itself is not flat, it is difficult to make an adequate connection between the heat sink and the multilayer ceramic substrate.